In recent years, there is an increasing demand for networking electronic units in many fields including consumer electronics and society's infrastructure. The number of cables connecting between units tends to increase. On the other hand, the trend toward wireless continues in fields such as data transmission, for example, wireless LAN (Local Area Network). There is also an increasing demand for wireless supply of power. However, the wireless power supply technology is commercially available only for IH cooking heaters, shavers, and cordless telephones. Such supplies transmit power at very short distances, almost in a contacted state. Degraded transmission efficiency due to radio wave diffusion inhibits three-dimensional power transmission when dealing with distances of several meters or power levels of several watts or more. The rate of adoption of wireless power supply technology remains stagnant. Incorrect positioning greatly degrades characteristics of an electronic unit utilizing a commercially available wireless power supply. The electronic unit needs to be placed at a specified position with limited freedom for placement.
For example, Japanese Unexamined Patent Application Publication No. 2010-093446 discloses the planar propagation medium as a technology to solve these problems. The technology can transmit an electromagnetic wave between two planar conductors that sandwich a planar dielectric. One of the planar conductors is formed in a meshed pattern. The electromagnetic wave interface is provided via a thin-film dielectric. An evanescent wave leaking near the meshed conductor enables output and input of an electromagnetic wave. The same publication discloses a surface wave transmission system that propagates an electromagnetic wave (referred to as a surface wave) trapped in a planar propagation medium. This system two-dimensionally transmits power along the planar propagation medium and enables higher efficiency than three-dimensional transmission. Just placing a device to be powered on the planar propagation medium enables power transmission. The system ensures a high degree of freedom for placement and may serve as continuous power supply for mobile devices.
Japanese Unexamined Patent Application Publication No. 2009-188737 discloses the technology of miniaturizing an ordinary antenna used in free space. Dipole antennas are modified to provide two L-shaped dipole antennas. These antennas are symmetrically placed at opposite corners on a conductive plate. A power supply point is provided at the end of a signal conductor for each antenna.
Japanese Unexamined Patent Application Publication No. 2010-093446 uses electromagnetic wave coupling. Therefore, the size of an electromagnetic wave interface depends on wavelengths in the planar propagation medium and tends to increase. To solve this problem, the same publication discloses coupling a spiral conductor for the electromagnetic wave interface and a meshed conductor for the planar propagation medium using mutual inductance (M) between them. The distance between both conductors can be shortened, and the electromagnetic wave interface can be minimized without decreasing the mutual inductance. However, the mutual inductance varies due to the relative positions of the spiral conductor with respect to the meshed conductor, such that the power receiving amount is unstable. The technology disclosed in Japanese Unexamined Patent Application Publication No. 2010-093446 is not applicable for providing continuous power to mobile device where the positional dependence of power supply characteristics is important. For the purpose of positional dependence improvement or miniaturization, shortening a distance (Im) between lines on the spiral conductor causes additional problems, such as a decreased self-resonant frequency due to increased parasitic capacitance.
The miniaturization technology described in Japanese Unexamined Patent Application Publication No. 2009-188737 provides a planar antenna using two L-shaped dipole antennas placed opposite each other. The power is supplied from the center of the L-shaped dipole antennas. Theoretically, the miniaturization is limited to approximately a quarter wavelength. The linear antenna narrows an operating frequency band. The antenna cannot be easily miniaturized in combination with a power receiving circuit including a rectifier and a regulator.